1. Field of the Invention
The invention in general relates to bubble detection, and particularly to the detection of bubbles in the drilling mud of offshore drilling operations.
2. Description of the Prior Art
In the drilling of an oil or gas well, drilling fluid referred to in the industry as "mud," is pumped into the drill pipe where it proceeds out through the drill bit and up the annular space between the drill pipe and the walls of the hole. The purpose of the circulating mud is to clean, cool and lubricate the bit, flush to the surface the cuttings from the bore hole and to protect the walls of the hole until casing is inserted. The density of the mud is carefully controlled at the surface so as to contain various pressures encountered in the hole.
As a well is drilled into the vicinity of an oil deposit, gases may be released form porous rock and find their way into the circulating mud. The presence of such gas in the mud may modify the buoyancy of the drilling string and can cause extensive damage if it goes undetected. Instances have been recorded where drill pipe has been thrown straight up from the well with consequent extensive damage to the drill rig and other equipment when falling back to earth.
Blowouts have been known to cause disastrous fires, and in many instances the gases released may be noxious, such as hydrogen sulfide.
In offshore drilling, without proper controls, emerging gases may disrupt footings of rigs, causing capsizing, and in other instances floating vessels may actually sink since they cannot float on a layer of bubbles.
Presently, detection of down hole conditions is made by an examination of the circulating mud, at the surface. What is needed, however, is the detection of bubbles at a relatively early stage so as to allow corrective action to be immediately taken.
Several acoustic methods have been proposed for the detection of bubbles; however, not in the field of oil well drilling. For example, one system has been proposed for detecting bubbles from leaking containers having pressurized gas therein. The arrangement immerses the containers to be tested in a liquid and an acoustic doppler system is utilized for projecting acoustic energy into the liquid. The appearance of any bubbles from a leaking container causes a doppler frequency shift, indicative of the leak. Such a system, however, for use in conjunction with an oil well would not operate satisfactorily since the presence of solid particulate matter being carried by the drilling mud would cause a doppler readout, even without the presence of gas bubbles.